Since Si steel sheets are higher in magnetic permeability and electric resistance in comparison with electrical steel sheets containing no Si, and may be produced economically, those have been mass-produced as magnetic cores for electric power. It is known that Si steel sheet shows that the more is Si content, the better is the soft magnetic characteristic, and it shows a peak at 6.5% Si.
However, since, if Si content were more than 4.0%, an elongation would be rapidly lowered, ordinary cold rolling could not be carried out. Therefore, it has been industrially difficult to produce the thin sheets of high Si-Fe alloy containing Si more than 3%. With respect to such difficulties, it is reported that if hot rolling conditions are appropriately selected after hot forging, cold rolling would be possible to a certain extent (Ishizaka et al: Journal of Japan Institute of Metals Vol 30 (1966) No. 6).
This report teaches, melting the alloys containing 1 to 7% Si by high frequency induction furnace in the air so as to produce ingots of 50 mm square, hot forging the ingots until 15 mm thickness, machining these ingot test pieces on the surfaces until coming to 11 mm thickness, hot rolling them until 1 mm thickness at the temperatures of 1000.degree., 850.degree. and 750.degree. C. respectively; or hot rolling the samples until 5 mm thickness at 750.degree. C., followed by hot-rolling until 1 mm thickness at 600.degree. C., and hot rolling until 5 mm thickness 750.degree. C., followed by hot rolling until 3 mm thickness at 600.degree. C., hot rolling the samples until 1 mm thickness at 450.degree. C.; and pickling and cold rolling them for observing appearance of crackings, in order to investigate influences of the hot rolling conditions to the cold workability. According to this report, at Si content of not more than about 4.7%, the cold rolling is possible, irrespectively of the hot rolling conditions, and at about 5% Si the cold rolling is also possible, irrespectively of the hot rolling conditions, if the edge of the hot rolled sheet is removed by machining. However in the steel sheet of more than about 6% Si, the cold rolling property thereafter depends upon the hot rolling temperatures, and especially the steel of around 6.5% Si may be cold rolled by performing the hot rolling at the temperatures of 600.degree. to 750.degree. C.
On the other hand, there is a rapid solidification process (cooling rate is ordinarily more than 10.sup.5 .degree. C./sec) for making the thin sheets, other than the above mentioned rolling procedures (for example, Patent Specification Laid Open No. 16926/84).
Since the former of said procedures necessarily requires the hot forging before the rolling, the process cannot but be discontinuous due to presence of the hot forging, resulting in the complicated process and the high production cost. Further, if the cast ingot is subjected to the hot forging, cracks are generated, and therefore surface-machining prior to the hot rolling is necessary. In fact, the experiment in said report carried out the machining of about 27% (15 mm thickness to 11 mm) for the surface treatment. In addition, for rolling the steel at the temperatures of less than 750.degree. C. where the cold rolling property was excellent, the steel could not be rolled directly at this temperature, and the rolling was involved with inconvenienses of undertaking a pre-rolling more than 750.degree. C., followed by a subsequent rolling. As is seen, it is very difficult to practise the above mentioned processes in the industrial scale in view of the production cost and yield.
The latter of the rapid solidification process spouts the molten metal from the nozzle to the surface of a cooling roller and solidifies it, and is possible to produce thin plates continuously and at high yield. In this case, the maxiumum thickness is about 100.mu., and the width is about 20 cm at the maximum. Therefore the usage is limited, and the production in the industrial scale has not yet been realized.